It works!

Yay, I received my Pi at last!

Following John's good precedent, I measured my polyfuse resistances to be:

F1 = 4.0 ohms

F2 = 3.2 ohms

F3 = 0.3 ohms (but maybe close to 0.2, occasional flicker), LS digit questionable

These values are a lot higher than John's.  I used two different DVMs, one of them a costly Fluke (although not calibrated), but neither has probe lead resistance nulling (I subtracted the 0.2 and 0.3 ohms indicated when the probes are shorted).

Allowing for the lead offsets, the two DVMs track each other perfectly across this range, so I have confidence in the results.  I don't regard the F3 reading as reliable, since it's on the least significant digit displayed.  It's too high for comfort though (0.5 and 0.6 ohm readings on the DVMs) --- in effect the board's +5V is unregulated because of it.

F1 and F2 are outrageously high of course, and do more harm than good by deregulating the supply even further.  This is a plain and simple design fault.  Also, the fact that F1 and F2 have so greatly differing resistances is a worry --- clearly this component does not have a tightly defined resistance parameter, even worse when compared with John's.

I haven't powered the board up yet, as I must be the only person on the planet without a micro-USB charger.

I'll pop out and buy one today, or an adapter for a barrel connector or something.

Morgaine.

Yay, I received my Pi at last!

Following John's good precedent, I measured my polyfuse resistances to be:

F1 = 4.0 ohms

F2 = 3.2 ohms

F3 = 0.3 ohms (but maybe close to 0.2, occasional flicker), LS digit questionable

These values are a lot higher than John's.  I used two different DVMs, one of them a costly Fluke (although not calibrated), but neither has probe lead resistance nulling (I subtracted the 0.2 and 0.3 ohms indicated when the probes are shorted).

Allowing for the lead offsets, the two DVMs track each other perfectly across this range, so I have confidence in the results.  I don't regard the F3 reading as reliable, since it's on the least significant digit displayed.  It's too high for comfort though (0.5 and 0.6 ohm readings on the DVMs) --- in effect the board's +5V is unregulated because of it.

F1 and F2 are outrageously high of course, and do more harm than good by deregulating the supply even further.  This is a plain and simple design fault.  Also, the fact that F1 and F2 have so greatly differing resistances is a worry --- clearly this component does not have a tightly defined resistance parameter, even worse when compared with John's.

I haven't powered the board up yet, as I must be the only person on the planet without a micro-USB charger.

I'll pop out and buy one today, or an adapter for a barrel connector or something.

Morgaine.

Haha! so much time on the forums and then didn't prepare for the coming of the 'pi.

There are way to many people who say things like: this won't work. just from a theoretical viewpoint.

0.3 ohms at max 0.7A will give you 0.2V maximum drop.

With a proper regulated 5.0V powersupply, that will give you 4.8 to 5.0V on the line labeled "+5V" on the raspberry pi.

Why the F*** do you think that won't work? The 5V is connected to the 3.3V LDO. That will work downto 4.3V AT LEAST (i.e. at maximum load on the regulator, but the 'pi is using less). The 5V is connected to the line labeled "VBAT" on the BCM2835. That leads me to suspect that this is intended to work on a single cell LIPO. So it should work downto about 3.0V, but AT LEAST to 3.3V.

Next it goes to the USB ports. If you read the USB specs, devices are required to work down to somewhere in the low 4V. Not all do, but they are not USB compliant. They migth work on lots of other USB ports that do deliver a higher voltage, but the remain out of spec.

So with a worst-case 4.8V (4.9 in practise) at the top of F1 and F2, and a 100mA max powerdraw, we'd loose another 0.4V with the 4 ohms on those fuses. That's indeed close to spec.

If an USB device doesn't work, you have several options.

* Get a different device that does work.

* Get a powered hub in between.

* short out F1 and/or F2.

* if that doesn't help enough, short out F3 as well.

When you start shorting out those fuses, you'll be relying on the current limiting on the power supply. No big deal. Connect it to a PC powersupply and start shorting out things on the GPIO port and you'll be able to start a fire. Don't do that then!

Roger Wolff wrote:

 

With a proper regulated 5.0V powersupply, that will give you 4.8 to 5.0V on the line labeled "+5V" on the raspberry pi.

And what exactly does a "proper regulated 5.0V powersupply" have to do with a USB cellphone charger?  Virtually nothing in practice.  It's no use talking theory, or saying there's a proper spec for USB chargers, when massive experience in the Pi community has shown that cellphone chargers are very commonly unadulterated junk, and very frequently you have to try several before you find one that works at all even when they claim adequate amperage.

It's wrong to think of it as a "power supply", which conjures up ideas of stability, regulation, protection, and even some reasonable headroom.  It's just a charger, and it's made to the lowest possible spec and with the worst quality materials they can get away with.  The cellphone market is totally cutthroat and cellphones are given away free with phone contracts, so it doesn't surprise me in the slightest that the situation is as bad as it is.

The end result is not a supply voltage you can rely on, and making the situation worse with extra drop across a 0.3 ohm polyfuse is not clever when the Pi is specifically targetted at using cellphone chargers as its power source owing to the micro-USB connector.

Why the F*** do you think that won't work?

FUSE?

To answer seriously though, it will work in many cases, but it's marginal ,and it's poor design because it causes substantial loss of regulation.

The worst part of this though is the coupling together of distinct power paths, totally unnecessarily.  Nothing you connect to the Pi can expect a stable voltage source even if you provide one at the micro-USB input, because F3 deregulates it for all sinks on the board, including the two USBs and the P1 header.  Under no circumstances can the Pi's power design be considered satisfactory when it turns a regulated supply into a highly unregulated one.

Just because the nominal +5V for the two USBs might stay within USB spec (if you're lucky), this doesn't mean that it's good power design for something that is powered by P1-pin2 to experience a sizeable voltage drop whenever USB usage changes.  There's nothing wrong with using cheap polyfuses, but the power paths should not be coupled in this way.  F3 should control the SoC alone, F1 and F2 should tap off the power input connector, and P1-pin2 should have an independent polyfuse also tapping directly off the input connector.  F3 as it stands is a misdesign, and all the trip values are too low.

Morgaine.

Morgaine Dinova wrote:

 

And what exactly does a "proper regulated 5.0V powersupply" have to do with a USB cellphone charger?  Virtually nothing in practice.  It's no use talking theory, or saying there's a proper spec for USB chargers, when massive experience in the Pi community has shown that cellphone chargers are very commonly unadulterated junk, and very frequently you have to try several before you find one that works at all even when they claim adequate amperage.

Nobody has convinced me of such a thing. Before the first raspberry pi's were being delivered, I started making noises that even though the power supplies might be up to spec, most cables are not.

We have measured whole ohms in cables we had lying around.

Of course, people have reported that when they got a better USB-charger the pi suddenly started working. But with the high quality charger, they also got a better cable. So it's quite possible they used the new cable as well. And of course they stopped testing the moment the 'pi started working. So it's the new usb charger that did the job.

Pete Lomas who, as far as I know, worked on both the schematics and the layout of the 'pi. Has his habits. He's overly cautios about overvoltages, decoupling capacitors, and fuses. So there are a bunch of "extra" capacitors on the board that wouldn't have been necessary. There are a bunch of clamping diodes that 99% of the users will never use. And there are a bunch of fuses that some (including me) think are redundant.

And a 0.14V voltage drop in practice is the consequence of the fuse. So what?

Followup to measuring the polyfuses and shopping for a few means of applying power to the Pi ...

Short version:  everything worked first time, but the Pi is piping hot! 

Longer version:  Maplins came up trumps with lots of options for micro-USB power.  This one ---http://www.maplin.co.uk/micro-usb-power-supply-393067  --- even says "Can be used with the Raspberry Pi system" on the product page, lol.  Everyone is getting in on the Pi bandwagon.

That 1A power supply (which does seem to deserve the name, rather than merely "charger") supplies my Pi with 5.01V at the input to polyfuse F3, and I see 4.89V at TP1, when running a basic system load comprised of a Logitech universal receiver which runs totally cold, Ethernet, and HDMI into the HDMI input on a Dell 2408 monitor at 1920x1200.

Pulling out the Ethernet cable raised TP1 by 30mV without any significant change at the input.to F3, so this supply is regulating and its lead seems to be very good.  Pulling out the Logitech receiver raises TP1 by only 10mV.  Pulling out the HDMI lead has no effect on TP1.  My peripherals are taking very little current.

Under these condition, the finger test applied to the SoC/PoP (which BTW is a Samsung) has to be removed in under 10 seconds to avoid pain.  Applied to the LAN chip, the finger has to be removed almost instantly --- this device is running very hot indeed, both with and without the LAN cable.

Nothing else to report really.  I don't know Debian specifically but experience from many other distros made everything easy.  The nearest thing to a hiccup was that "apt-get install gpm" failed to configure the mouse, so I had to modify /etc/gpm.conf slightly before it would work (changed the mouse device to /dev/input/mice).

As an aside, I also bought http://www.maplin.co.uk/high-power-usb-charger-with-travel-adaptors-513509 on the expectation that its 2.1 A rating would quite likely offer better regulation, but that's not what I observed:  it delivers 4.75V at TP1 under the same load described above.  The blame might lie with the lead rather than the charger (haven't checked yet), but that's not what appearances would suggest as the lead is nearly twice as thick as the one on the better supply.  Of course, the extra thickness could be all plastic.

So, a successful start with my Pi, but running the LAN9512 that hot is a bit worrying.

Morgaine.

Morgaine Dinova wrote:

 

Under these condition, the finger test applied to the SoC/PoP (which BTW is a Samsung) has to be removed in under 10 seconds to avoid pain.  Applied to the LAN chip, the finger has to be removed almost instantly --- this device is running very hot indeed, both with and without the LAN cable.

 

So, a successful start with my Pi, but running the LAN9512 that hot is a bit worrying.

 

Morgaine.

Thank you for the write-up.  It's interesting that the RasPi forum doesn't seem to have a place to write up success, only problems :-)

It would be interesting to find out the LAN9512 temperature range seen in the field.  Some people have reported blazing hot as you have, others something more reasonable.  My fear regarding the LAN9512 is that its thermal performance may depend heavily on how well its large ground pad is soldered, which is impossible to see without an X-ray (if then).  How hot is the back side of your board?  When I first picked up my RasPi after running her the first time, the back side (which was sitting on the anti-static bag) was very hot. That's when I decided to get out some tools and make my vertical mount frame.  Radiating from both sides keeps her very comfy.

My Pi was one of the first released batch and my LAN chip gets warm but not hot, which makes me wonder if there has been any component changes or if some manufacturing defect such as partial shorts or poor soldering have it the later batches.

There have been some apparently minor differences in the markings on

the LAN chip, but nothing correlated with temperature. 

The thread "Ethernet chip gets raging hot" started on May 15,

http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=5898

so this isn't a particularly recent issue.

As far as I know, the only official comment was from Gert in the

above thread, who wrote:

"It might be warm, it should never be hot."

and later wrote:

"I can imagine that you do not want to give up your Pi now. But when a few more are available I would really send it back so somebody can find out why it gets so hot."

So maybe you will be the first person to take them up on

this offer, now that we know replacements are readily

available.  They really ought to publish what they consider

to be the normal temperature range, as well as what they

consider to be the normal range for polyfuse resistance.

Measuring the current into your 'pi is a bit more difficult than, measuring the voltage across TP1,TP2.

But I'm guessing you're seeing the same as me, judging from the voltages you measure across F3. We're using the "has too much resistance" fuse as a current sense resistor!

My 'pi has 140mV across F3. Yours has only 120mv. Either your polyfuse is a bit better, or your 'pi is using slightly less power. But in the grand scheme of things: Your pi is using the same amount of power as mine.

All power used gets converted into heat. Your pi is not producing more heat than mine.

The only think I can think of that would make the lan9512 hotter than it should be is if your 3.3V is WAY too high. We measured 3.31V on ours the other day. Measure it, the tab is 3.3V, TP2 is GND.

On the other hand, that would increase total power usage....

The thermal resistance between where the power is generated and the outside of  the package is much less for the lan than for the soc. That might lead to a percieved temperature difference.

Put your finger on the chip, when it becomes uncomfortable, swtich fingers. I'm guessing that the second or third finger won't have any trouble staying on the chip. Your finger is now acting as a heatsink. At first the thermal capacity of the package was heating the finger beyond "comfortable", but after getting rid of that heat, if your finger is able to dissipate the heat, it's less than 1W.

Or at least, with my fingers its less than 1W. That's what I measured once.

The power on the 'pi shares between the 3.3V regulator, the soc and the lan chip. And input power is about 2.4W, so at most 1W for the lan and soc and a little less for the 3.3V.

@coder27: "Raging hot" describes the temperature of my LAN9512 perfectly too, so if Gert says that it should not be so, it's faulty.

@Roger: I did your suggested finger-switching to drain the thermal capacity of the chip surface, and you're right, it has only a small thermal reservoir.  Using up to 4 fingers in sequence, the 1st has to be removed as fast as human reactions allow (it's not charring, but seriously beyond cell survival temperature), the 2nd maybe 1/5th second, the 3rd a whole second, and the 4th up to 5 seconds.  I can't extend "Finger Contact Survival Time" any longer than 5 seconds no matter how many times I switch, I'm not into pain.

In the absence of a thermometer, I must say I like the FCST concept.

The chip might survive this operating temperature, but I doubt that the board will last long like this because of the physical stresses of expansion/contraction whenever the board is powered up or down.

Talking to RPF about this is a dead loss as they don't admit any problems (I can't anyway, having been banned for talking politely to Liz), but I'd like to know what Element 14 / Farnell's position is on this.  Should we try to extend board life by heatsinking the LAN9512 chip, or return it as faulty if Gert says that such high temperature is not expected?

Morgaine.

Morgaine, I just did the "finger switching" test again as I suggested. :-)

It seems my LAN chip is a bit hotter than I remember. I too can't get much beyond 5sec wtihout hurting my finger. It could also be that the outside temperature is now 25 degrees, while before it may only have been 20 degrees.

The "can't keep finger on" temperature is pretty much 52 degrees C. No matter who you are or if you say you have fire resistant fingers.

If the outside temperature rises, more heat will need to flow through your finger (less is being diverted by the PCB), and the hotter it gets. Even a 2 degree change might mean two out of two people now report unable to hold beyond 5 sec, while before it was "possible to hold indefinitively"

If you look at the datasheet, on page 41, it says you can expect a typical power use of about 763 mW.

http://www.smsc.com/media/Downloads_Public/Data_Sheets/9512.pdf

I don't pretend to be able to estimate the power-use within  a factor of two, so when I say "about 1W", this is certainly within range.

Regretably the thermal resistance isn't specified. The Texas instruments VCA8500 provides a "typical" value (i.e. a wild guess that it might apply to the lan9512), of 22 degrees / W. At 0.7W, that would mean only about 40 degrees C. We're seeing more than that.

But if the wild guess is off-by-a-factor-of-two, and it's 40 degrees/C for teh lan9512, then 60 degrees sounds like a possible value.

With the case at around sixty degrees, the inside will be only about 80 degrees. This is well below the maximum of 150 degrees.

Chips are built to work at these temperatures. Sure, I too have heard about some chips working themselves loose from thermal stresses. Before you complain to Farnell, the RPF or me, please wait until you KNOW it will fail because it HAS failed.

Some chips use a bit of power, and therefore they dissipate heat. That heat needs to be carried away and a temperature gradient is required before heat starts moving. This is simply the way things work.

Now usually these things are hidden inside a case. So you don't get to poke your fingers at it. The 'pi is open, As a learining experience YOU get to poke your fingers at it. Have you learned something today? That's the stated primary goal for the RPF.